1. Field
The following description relates to an optical fiber, and more particularly, to an optical fiber that has an extremely low bending loss.
2. Description of Related Art
Fiber to the x (FTTx) is a collective term for various optical fiber delivery topologies that are categorized according to where the fiber terminates. Recently, along with continuous expanding of the Fiber to the x (FTTx) market, various countries are investing in infrastructure including FTTx. For example, due to the increase in the amount of use of wireless data by smart phones, the wide spread of Three-Dimensional (3D) TVs, the introduction of TV portable services, the growth of wireless Internet Fourth-Generation (4G), such as Long Term Evolution (LTE) and WiMAX, and the like, there has been a required increase in the capacity of an optical backbone network.
Efficient network installation and operation of the FTTx depend on environmental restraints of a cable spreading site. Accordingly, service operators and optical fiber providers are continuously attempting to overcome a bending loss that is generated by a physical force at the site, especially, in an extreme bending environment.
To prevent degradation from bending loss of a single-mode optical fiber, a Mode-Field Diameter (MFD) control technique, a depressed clad manufacturing technique, a low-index trench manufacturing technique, a manufacturing technique for a nano-size air hole having an isotropy ring structure in the clad, and the like, have been proposed. However, commercialized techniques capable of maintaining the compatibility of connection with existing single-mode optical fibers (ITU-T G.652.D) while a the same time satisfying optical characteristics and reliability specifications of the international standard ITU-T G.656.B3, are extremely limited.
In an attempt to satisfy the foregoing optical characteristics and reliability characteristics, a manufacturing technique has been commercialized which includes a low-index trench including a core, an inner layer, and a minimum-index trench layer. Because the low-index trench technique has a superior mechanical and environmental reliability with a solid glass structure and because it is suitable for mass production, the low-index trench manufacturing technique has attracted much attention as a method for implementing an extreme bend insensitive fiber. Recently, as bending characteristics required in the spreading site have become more severe, an effort has been made to reinforce the bending loss characteristics by forming a deep trench.
However, one difficulty with the low-index trench manufacturing technique is minimizing the bending loss while maintaining a short cutoff wavelength. The characteristics of the cutoff wavelength in the optical fiber are inversely proportional to the bending loss characteristics. Accordingly, a high-order mode becomes difficult to control as the bending characteristics are strengthened. For example, a high-order mode such as LP11 or higher, which guides waves in the optical fiber, is more concentrated due to an interfacial condition of a deep trench region, i.e., an index difference, such that the waves are guided across a long distance while the high-order mode characteristics are maintained.
To address the foregoing problems, control over doping concentrations of the core and the trench layer and the sizes of the core and the trench layer are performed which typically degrades the yield in the actual manufacturing process.